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Biomedical subjects

K Thyagarajan

Publications and source records attributed to K Thyagarajan.

At least 19 recordsLinked to original sources
PubMed2005

Broadband, lossless, dispersion-compensating asymmetrical twin-core fiber design with flat-gain Raman amplification.

We report here a broadband, lossless, dispersion-compensating asymmetrical twin-core fiber design with flat-gain Raman amplification that uses a single pump. Simulations show that broadband Raman amplification, with +/- 0.1-dB gain ripple, is achievable over a 31-nm bandwidth (1504-1535 nm) by use of a single pump. Amplifier characteristics have been modeled, with the effects of wavelength-dependent splice and background attenuation loss taken into account. The fiber also has a high negative-dispersion coefficient [-230 to -330 ps/(km nm)] over the operating wavelength range and, hence, only 12.5 km of this fiber can compensate for an accumulated dispersion of 240 km of standard transmission fiber. The device is thus proposed as a lossless dispersion-compensating module wherein lossless operation is achieved by use of inherently gain-flattened Raman amplification.

Journal Article↗
PubMed2002

Evaluation of the phenol ammonium sulfate sedimentation smear microscopy method for diagnosis of pulmonary tuberculosis.

We compared the sensitivity and specificity of the phenol ammonium sulfate (PhAS) sediment smear microscopy method for detection of acid-fast bacilli with those of direct smear microscopy, using culture results for Mycobacterium tuberculosis as the "gold standard." The sensitivities of the PhAS and direct smear methods were 85% (465 of 547) and 83% (454 of 547), respectively, and the specificity of each method was 97%. The PhAS method was better accepted by the laboratory technicians and safer but necessitates an overnight sedimentation, which delays reporting of results until 1 day after sputum collection.

Ammonium Sulfate↗
PubMed1996

Surface and edge wear of Björk-Shiley Delrin heart valve discs.

BACKGROUND AND AIMS OF THE STUDY: Wear of Björk-Shiley Delrin (BSD) heart valve discs is known to have occurred in some patients, possibly contributing to increased regurgitation. This paper specifically addresses surface and edge wear that have been observed on some discs of explanted BSD valves after implant durations up to 22.4 years. METHODS: The wear patterns have been documented using either photographic or scanning electron microscopic methods for 42 out of 73 explanted BSD valve discs. The remainder of the 73 discs were not available for analysis. RESULTS: One form of surface wear found on 18 out of 42 of the Delrin discs was the concentric wear of mild abrasive origin along the surface near the disc edge due to contact with the inlet and outlet struts. In five instances, surface anomalies were observed, primarily in the areas of high velocity blood flow. This paper also describes two Delrin discs with non-concentric edge wear patterns: (a) one which appears to be due to fatigue micro-chipping and abrasive wear of the disc of a 20 year BSD explant, which had fibrous tissue ingrowth, causing abnormal rotation of the disc during valve closure, and (b) a second one which is thought to have been caused by a cutting action of the knife-like stub of one inlet strut leg which had separated. Cross-sectional analyses of two explanted BSD discs, with full indent grooves on the inflow side, indicated that the Delrin material was primarily compressed under these wear grooves, rather than removed by abrasion. Hardness profiles indicated that the Delrin 150 microns below the surface was harder and would tend to prevent further deformation. A simple model describing the compound impact (impact with sliding) phenomenon is introduced to explain abrasive wear found on some explanted BSD discs. CONCLUSION: Based on the studies here and reports in the literature, the BSD heart valve appears to present a design which provides many years of service and, when wear occurs, it occurs in a manner that provides easily recognized clinical symptoms, which allow time for diagnosis and treatment.

Aortic Valve↗
PubMed1996

A comparison of wear patterns in vivo and in vitro for Björk-Shiley Delrin heart valve discs.

MATERIALS AND METHODS: Wear patterns for 33 explanted Björk-Shiley Delrin heart valve discs with known implant durations were compared and contrasted with the wear patterns for 17 discs from accelerated wear tested (AWT) valves. Previous test reports and the current study provided data for the maximum depth of wear grooves for a total of 56 explanted BSD valve discs with implant durations up to 22.4 years. RESULTS: The discs of the explanted BSD valves showed, in general, more than one type of wear pattern. Eighteen out of 33 exhibited a full indent groove on the inflow side and 24 of the 33 discs showed well indents on the outflow side. Only two discs showed evidence of non-concentric edge wear. All maintained their original surface polish outside of these areas. The data indicated that wear patterns in the case of the AWT discs were more pronounced, and in some discs were different in type from those of discs from explanted valves. For example, no cracks, fractures or perforations of the Delrin discs were observed, or reported, for the explanted valves, while in the 17 AWT valves there were three cracked discs, four disc fractures, three disc well perforations and three perforating edge indents. CONCLUSION: It appears that the Shiley AWT conditions were more severe and imposed greater loads than those produced by patients with BSD valves implanted.

Biocompatible Materials↗
PubMed1996

Evaluation of disc retention of Björk-Shiley Delrin heart valves.

BACKGROUND AND AIMS OF THE STUDY: Impact wear grooves have been observed in the occluder discs of clinically explanted Björk-Shiley Delrin (BSD) heart valves due to repetitive closing impacts against the inlet strut. We examined whether this impact wear could increase the disc-to-strut gap, which could in turn affect the retention of occluder disc. MATERIALS AND METHODS: Three BSD heart valves with deep wear grooves were tested in a left ventricular assist device set-up simulating dynamic conditions that would facilitate the dislodgement. RESULTS: The disc was found to be rotating within the flange during the testing and no embolization of the occluder disc occurred in any of the BSD valves tested. The Björk retention limit and the physical retention limit were calculated for each valve. The Björk retention limit is defined as (disc well depth)-(disc-to strut gap)-(1/2 outlet strut wire diameter) and the physical retention as (disc well depth)-(disc-to strut gap). CONCLUSION: The disc retention would not be affected by the impact wear grooves as long as the disc-to-strut gap did not exceed the physical retention limit.

Biocompatible Materials↗
PubMed1994

Fixation of bioprosthetic tissues with monofunctional and multifunctional polyepoxy compounds.

Collagen from a native tissue is fixed with a polyepoxy compound (PC) for use as a new biologic prosthetic material. Prior studies have shown that this biomaterial has comparable properties with collagen fixed with glutaraldehyde (GA), and thus has great promise for biomedical applications. A prior kinetic study indicated that the reaction between the functional groups of collagen and the multifunctional epoxy EX-313 is a 2.5th-order reaction. The purpose of this study was to understand the mechanism of the amino acid-PC reactions in a fixation process. Bovine arteries were fixed with a monofunctional PC (EX-131) and a multifunctional PC (EX-313) as a function of fixation time. A sequential fixation with a second fixative was used to identify the available remaining reactive sites from a prior fixation. The denaturation temperature (Td) was measured on each sample. Because the denaturation temperature is a direct indication of crosslinking of individual amino acids with the fixative, the increase in Td of a subsequent fixation may be indicative of the available remaining amino acids. The fixation index was measured on each sample to reflect the increase of fixation completion in a sequential fixation process. The fixation index and crosslink data also revealed that the reactive amino acids for EX-131 and EX-313 may not be exactly the same. The data in this study suggest that a monofunctional fixative can pre-react with the amino acids of collagen to effectively block further fixation of collagen with a second fixative. This amino acid masking may be associated with collagen branching. Collagen branching and its effect on denaturation temperature are described.

Animals↗
PubMed1993

Kinetic study of collagen fixation with polyepoxy fixatives.

A new biomaterial has been developed by fixing native collagen with a polyepoxy compound (PC) fixative. In this study, bovine internal thoracic arteries were fixed with PC under various conditions to help understand the kinetics of the collagen-PC reactions and optimize the fixation process. At predetermined time intervals, small samples were cut from the arteries to determine the quantities of the remaining unreacted amino acids in the collagen. Temperature, concentration, and solution pH were among the key parameters studied. The overall fixation rate was found to be reaction-rate controlled, as the rate of fixation was relatively slow compared with the rate of diffusion of PC. As might be expected, the reaction rate was favored by a higher temperature, concentration, and solution pH. A kinetic model, with a 2.5th reaction order with respect to the reactive functional groups of collagen and a first order with respect to PC, was developed that gave a good fit to the experimental data. Based on this model, the degree of fixation, X, as a function of time, t, is given by (1 - X)-1.5 = 1 + Kt, where K is a constant related to the initial concentrations and the reaction rate constant.

Amino Acids↗
PubMed1993

Hemodynamic evaluation of a bioprosthetic venous prosthesis.

PURPOSE: A prosthetic venous valve must be biocompatible and nonthrombogenic and function in the venous circulation. Biocompatibility and thrombogenicity of our prosthesis have been examined in prior animal experiments, and 91% of valve conduits including early prototypes are patent at 3 weeks. However, evaluation of valve function is much more difficult in animals; therefore in this study the function of excised valves was evaluated ex vivo. METHODS: Nine bovine jugular vein conduits, each with one bileaflet venous valve, were harvested and placed in a venous flow simulator. Flows and pressures were adjusted to mimic human respiratory and hydrostatic variations. Each valve and conduit was tested for variations in valve diameter and sinus expansion in response to flow. Valve opening and closing times and valve competence were measured in response to pressure changes. After testing, each specimen was glutaraldehyde fixed and assessed a second time. RESULTS: Valve orifice area increased in response to flow in both fresh and fixed tissues. Maximum valve orifice area was reduced by fixation (27.7%) at full flows (p < 0.05). Valve sinus dimensions increased in response to increased pressure until maximum expansion was achieved (33 mm Hg). This was reduced 15.3% in fixed tissue (p < 0.05). Valve opening times (at < 1 mm Hg gradient) were slightly longer in fixed compared with fresh tissue (0.43 +/- 0.09 vs 0.41 +/- 0.13 second; p < 0.05). Valve closing times were comparable in both states (0.43 +/- 0.08 vs 0.49 +/- 0.07 second). Three fresh and seven fixed specimens that were subjected to 287 mm Hg back pressure exhibited minimal reflux. CONCLUSIONS: Size and availability make the bovine jugular vein valve an ideal venous valve substitute. Glutaraldehyde fixation renders the tissue biocompatible and nonthrombogenic while preserving anatomic integrity and leaflet strength and flexibility. Mounted and stented in a sewing sleeve, this prosthesis could represent the first generally applicable clinical solution to chronic venous insufficiency and venous hypertension.

Adult↗
PubMed1993

Development and evaluation of a pliable biological valved conduit. Part I: Preparation, biochemical properties, and histological findings.

Different types of external valved conduits have been used for the repair of complex congenital cardiac anomalies that may have otherwise been inoperable. However, an ideal conduit has yet to be found due to complications such as stenosis, thrombosis, calcification of the valve and graft wall, and "peeling" of the neointima. To address those problems, a new extracardiac valved conduit made of bovine jugular vein was developed and evaluated in a preliminary animal study. Harvested bovine vein containing a naturally existing valve was initially incorporated with protamine on the inner surface and then was cross-linked in diglycidyl ether (DE). Fixation with DE allowed the vein and its leaflets to retain a tissue-like elasticity. To provide antithrombogenicity to the graft, heparin was introduced into the lumen to bind ionically to the pre-entrapped protamine. The biological valved conduit of approximately 14 mm diameter was implanted from the right ventricle to pulmonary artery as bypass graft in three dogs. After implantation, the native main pulmonary artery was ligated between the anastomotic sites of the bypass conduit. No anticoagulant or antiplatelet drugs were administered after surgery. One DE-fixed valved conduit was retrieved at 3 months, and the others were removed at 5 months. Only small thrombus areas were found on the white luminal surfaces. The valves and the conduits maintained softness and pliability, similar to before implantation. Additionally, the collagen content, shrink temperature, and tanning index of this newly developed biological valved conduit before and after fixation were measured in the study.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Heparinization of biological vascular graft reduces fibrin deposition.

An alternative graft is needed for coronary bypass operations in patients lacking suitable autologous vessels. We therefore studied Denaflex, a biologic graft, in a dog ex-vivo shunt model to determine whether heparin treatment makes this graft less thrombogenic. Comparison was also made to Bioflow, a nonheparinized biologic graft. Fibrinogen deposition during high flow (593 +/- 202 ml/min) decreased from 672 +/- 467 ng/mm2 in nonheparinized Denaflex grafts to 448 +/- 298 ng/mm2 (p < 0.05) in heparinized Denaflex grafts. At low flow (117 +/- 13 ml/min), heparinization of Denaflex grafts similarly decreased fibrinogen deposition from 1102 +/- 601 ng/mm2 to 703 +/- 405 ng/mm2 (p < 0.05). At both flow rates fibrinogen deposition in Bioflow grafts was less than in nonheparinized Denaflex, but was similar to heparinized Denaflex grafts. Platelet deposition was not influenced by heparinization of Denaflex grafts and was similar among Denaflex and Bioflow preparations. Whether Denaflex performs acceptably in vivo as a xenograft requires extensive study.

Animals↗
PubMed1993

Evaluation of collagen modification and surface properties of a bovine artery via polyepoxy compound fixation.

Collagen of bovine internal thoracic artery (BITA) was treated with glutaraldehyde (GA) or polyepoxy compounds (PC). This study was to evaluate the surface properties as a result of tissue tanning reaction with PC. The fixation resulted in a significant reduction of available lysine, histidine, and other amino acid residues in PC fixed grafts as compared to fresh pre-fixed arteries. Among them, the lysine (Lys) content was reduced by about 80%, indicating that PC reactions mainly involve with Lys residues. Both PC and GA treatment led to crosslinking as evidenced by the increase in the denaturation temperature. The critical surface tension and the Fourier Transform Infrared Spectrum (FTIR) on a pre-implant and its 96 days explant were evaluated and found to be similar. The FTIR analysis of a pre-implant and the 96 day explant indicated that there was no lipid deposition.

Amino Acids↗
PubMed1992

Evaluation of two epoxy ether compounds for biocompatible potential.

Bovine arterial tissue exposed to two epoxy ether compounds (Denacol EX-313 and Denacol EX-810) was evaluated for its biocompatible potential by in vitro and in vivo test procedures. The battery of test procedures included percent Inhibition of Cell Growth, Medium Eluate Method (MEM), Agar Overlay (AO), Blood Compatibility, Acute Mouse Systemic Injection, Rabbit Intracutaneous Irritation, Rabbit Subcutaneous Implantation, Guinea Pig Maximization, and Ames Tests. The epoxy exposed tissue was found to be noncytotoxic, nonmutagenic, and biocompatible by the test methods employed. In addition, the maximum concentrations of the unreacted Denacol EX-313 and EX-810 solutions found to demonstrate noncytotoxic reactions by the MEM and AO procedures were identified as 55 and 60 ppm for the MEM and 150 and 200 ppm for the AO procedure, respectively. These studies suggest that Denacol EX-313 and EX-810 are acceptable solutions for the processing of implantable tissue provided the epoxy residuals remain below those levels found to be cytotoxic.

Animals↗
PubMed1979

Oxidation of fatty alcohols to acids in the caecum of a gourami (Trichogaster cosby).

Oxidation of fatty alcohols to acids in gourami caeca was investigated by measuring the reduction of NAD+ and the formation of labeled hexadecanoic acid from [1(-14)C]hexadecanol. Virtually all dehydrogenase activity is in the microsomal fraction. Maximal activity is obtained with NAD+ as cofactor whereas with NADP+ 60% of that activity is obtained. The enzyme is rather specific for long chain alcohols and 2 NADH are formed for each molecule of hexadecanol oxidized to acid. It is stabilized by mercaptoethanol, and completely inhibited by p-chloromercuribenzoate. The activity is optimal at pH 9.5. At higher pH, small amounts of aldehyde are found. The first reaction in the sequence, fatty alcohol leads to aldehyde leads to acid seems to occur under the more physiological condition at a much slower rate than the second reaction so that free aldehyde is not detected. Addition of palmitic acid indicated an uncompetitive product inhibition. The oxidation of alcohol to acid is reversible only to a very minor extent even in the presence of NADPH, CoA, ATP and Mg2+. Location, activity and properties of the enzyme are in agreement with the earlier observation from dietary experiments that in the gourami fatty alcohols of wax esters are oxidized to acids in the course of absorption.

Alcohol Oxidoreductases↗